I will defer to the engineers on that one. I only know this because Duke helped develop the O2 pre-breathe protocol.I'm not doubt you, but I find it odd that they would design for that much pressure. 21% is as safe as air on earth in regards to a fire and human survival. But they could have saved a lot and get the same results by running a lower pressure. .8 or .9 ATA. But that gets into spacecraft design. I don't have the meeting notes on why 1 ATA was chosen.
I did see the James Webb telescope a few months before it was shipped off and flung into space. But that one isn't trying to keep anyone alive.
Best regards,
DDM